Huh. TIL. That boggles the mind though. Makes me wonder where it came from, or if perhaps its source was somehow killed off or otherwise destroyed, or if perhaps there is a reservoir somewhere that we simply are not aware of?

There’s a fair amount of viruses that have no animal reservoir.

Either they evolved from a different human virus, or from an animal virus.

Measles for example evolved from a livestock virus called rinderpest and lost the ability to infect cattle as it became a specialized human virus.